Generally, electronic apparatus, such as a radio or a voice composite instrument, provided with loudspeakers use an inner-housed moving-coil speaker because of its superior frequency characteristic.
It has recently become desirable to make electronic apparatus having the above loudspeaker thinner.
Since it is structurally difficult, however, to make the moving-coil speaker, made thinner, it has been difficult to make the electronic apparatus thinner for this reason, a loudspeaker of a piezoelectric-driven type instead of the moving-coil loudspeaker has been drawing public attention.
A conventional piezoelectric loudspeaker meeting this requirement has been proposed, which, as shown in FIG. 1, comprises a diaphragm 12 stretched across a frame 11 and a sound-generator which comprises a metallic plate 13 and a piezoelectric ceramic plate 14 adhered thereto to form a piezoelectric unimorph structure, the sound generator being bound to the diaphragm 12.
A loudspeaker constructed in this manner places the sound-generator 15 in approximately a free vibration condition despite the fact that it is adhered to the diaphragm 12. As a result, the Q-factor of sharpness of resonance at the resonance point is relatively large and the frequency characteristic is far from flat as shown by the curve A in FIG. 3. This is undesirous for the loudspeaker.
Also, it is troublesome to adhere the sound-generator of unimorph structure to the diaphragm stretched across the frame, which makes mass production difficult.
In order to solve the above problem, a sound-generator of unimorph structure, which flexibly vibrates in the bending mode and is supported circumferentially directly by the frame without using the diaphragm, has also proposed.
Such construction has a good characteristic for a buzzer, but is as a loudspeaker because its characteristics are not satisfactory to cover a voice band.